It is generally known from the conventional art that in disturbances in the power grid an electrical supply to high-availability electric consumers is assured by means of a so-called uninterruptible power supply, abbreviated UPS. The uninterruptible power supply is realized in that in a failure of the power grid or network supply the electric consumers continue to be supplied by means of uninterruptible power supply devices without interruption.
According to the conventional art, the uninterruptible power supply to high-availability consumers with uninterruptible power supply devices is assured, which are distinguished by different redundancies.
For the uninterruptible power supply devices the backup power supply thus occurs by means of so-called electrical distributed power generating units.
The high-availability consumers comprise critical consumers who should be supplied without interruption and essential consumers who can continue to be supplied with short term interruptions of up to ca. 15 seconds. An uninterruptible supply for critical customers in the present case is also understood to supply power with short interruptions of much less than 20 ms.
Critical consumers are supplied by uninterruptible power supply units, called UPS units hereafter, of a different type, i.e., statically or dynamically. Said UPS units and essential consumers, which continue to be supplied with interruption, in the case of failure of the power grid or the network supply are supplied with electrical power by internal combustion reciprocating piston engines or other distributed power generating units.
To increase the overall availability, the distributed power generating units usually have a redundant structure. For economic reasons, the redundant structure is generally realized with an N+1 redundancy.
On this basis, there are isolated parallel-connected supply devices with parallel-connected diesel-dynamic uninterruptible power supply units. In the diesel-dynamic units, the uninterruptible power supply to the critical consumers occurs via a synchronous generator, which is decoupled by an electrical, inductive impedance between the electrical network and the electric consumer.
The synchronous generator is powered by a synchronous motor during network operation. It is assured by an energy storage device that during an interruption of the network, the system generates power and continues to supply the critical consumers until the start of the coupled diesel engine. After the diesel engine is started, it takes over powering the synchronous generator, which continues to supply the load without interruption.
U.S. 2008/0034256 A1 discloses an isolated-parallel device for uninterruptible power supply. The device comprises two diesel-operated UPS units, both of which work with a low voltage, medium voltage, or a combination of low and medium voltage. For the electrical supply to critical and essential consumers, the UPS units are provided with electrical power from a power grid. In this regard, the UPS units are designed for operation as independent systems, whereby each UPS unit comprises a choke coil for conditioning the electrical power obtained from the power grid for use for the critical consumers and a diesel engine, which is provided for the long-term backup supply of electrical power to the critical and essential consumers during interruption of the power grid supply. Furthermore, the UPS units in each case comprise a power storage device, which is coupled to the diesel engine generator unit and provides for short-term backup power supply to the critical consumers during the diesel engine start period. Furthermore, a system switch panel is provided for each UPS unit; said switching unit controls the electrical supply to the important and critical electrical loads during a change from grid power to the UPS unit. Furthermore, an output bus is provided for each UPS unit, the said bus controls the electrical connection of the choke coil and the diesel engine generator unit to the important and critical electrical loads and the transmission of electrical power from and to the power grid, from and to the diesel engine generator unit, and from and to the choke coil. Further, an adjustable choke coil, coupled to the output bus of a UPS unit, is provided, which allows energy flow from one output bus to another output bus and limits the fault current between the UPS units and thus isolates each UPS unit from the electrical faults of the other UPS units.
Furthermore, WO 98/09359 A1 discloses a device and a method for supplying electrical loads. The device comprises an electrically conductive distribution bus, which is coupled to two electrical power sources and the electrical loads. At least three fast acting fault current limiters are coupled to the distribution bus via which the generators and the electrical loads are connected. This ensures that faults occurring at generators or load connections are interrupted quickly enough to avoid compromising the common bus voltage and other connected consumers.
DE 10 2007 021 089 B3, which corresponds to U.S. Pat. No. 7,825,541, and which describes a device with multiple parallel-connected backup power sources, whereby the backup power sources are each connected in a detachable fashion on the input side to an AC grid via a choke coil and on the output side to a common load bus. The power, output by each individual backup power source, is adjustable. Further, a controller is provided, which when a failure in the AC grid occurs, disconnects the backup power sources affected by the failure from the AC grid. Further, the controller is provided to reconnect the backup power sources one after the other to the AC grid after the return of the AC grid and to coordinate the power output by the individual backup power sources. The controller has a device for determining a phase angle between the AC grid and the load bus. Further, to coordinate the power output by the individual backup power sources, while one part of the backup power sources is connected to the AC grid on the input side and another part of the backup power sources is not connected to the AC grid on the input side, the controller adjusts the power output by each of the backup power sources not yet reconnected to the AC grid as a function of the phase angle. Further, a method for controlling multiple parallel-connected backup power sources is described.